In the following, reference will be essentially to potentiometric sensors, as an example of a liquid, or gas, sensor. Potentiometric sensors are used above all for determining potentials at large resistances, such as is the case for pH-measurements and redox-measurements. With the help of pH-electrodes, or redox-electrodes, as the case may be, the potentials of solutions are sensed.
These electrodes are exposed to strong wear in many cases of application, so that they must frequently be replaced after a short time of operation.
There exist very simply constructed pH-sensors, which consist only of a pH-electrode, without any electronic components. These pH-electrodes deliver a pH-dependent potential, which can be accessed on suitable, electrical connections. Optionally, these pH-electrodes have an integrated temperature sensor, e.g. PT100, for temperature compensation. The potential of the temperature sensor can be accessed at suitable temperature outputs. For measuring, these pH-sensors are usually connected via a cable to a transmitter, which generates a measurement signal from the pH-dependent potential and, as required, from the temperature signal of the temperature sensor.
Besides the described, simple pH-electrodes, or sensors, there are also those with integrated preamplifier for impedance conversion. The output signal of the preamplifier is that potential of the pH-sensor, with, however, instead of the internal resistance of the pH-sensor, which lies in the order of magnitude of 100 M, now the internal resistance of the preamplifier of some few ohms being the determining factor. Consequently, the further transmission and processing of the output potential is considerably simplified for a transmitter. The preamplifier is either fed via a battery or supplied with voltage via a cable.
Furthermore, under the name Direct Line of the Honeywell company, simple transmitters are obtainable, which are mounted directly on the pH-sensors. In this way, it is possible to generate, in the immediate vicinity of the sensor, e.g. a 4-20 mA measurement signal, which can then be transmitted, without more, to the control room.
In the case of all known pH-electrodes, or pH-sensors, it is necessary to calibrate the electrodes after connection to the transmitter, in order to be able to store the determined calibration parameters in the transmitter. Sensor specific information, such as measuring point name, etc., are, as a rule, not obtainable on-site, thus in the immediate vicinity of the sensor.
Recently, a pH-sensor has become known (available from the firm of Endress+Hauser under the mark MemoSens), which is composed of a sensor module and a sensor module head, which can be plugged together. The data and energy transfer between sensor module and sensor module head occurs contactlessly via a connection zone, which serves for galvanic decoupling. Additionally, a digital memory is provided in the sensor module, for storing, among other things, calibration parameters.
Likewise, it is not possible to check, in simple manner, whether a pH-sensor is capable of functioning. In the control room, one must rely on a secure data transfer on the path from sensor to control room.